1. Field of the Invention
The invention concerns adhesive tape, preferably pressure-sensitive adhesive tape, the adhesive layer of which contains particles affording electrical conductivity through the thickness of the layer while being electrically insulating in lateral directions.
2. Description of the Related Art
As is pointed out in U.S. Pat. No. 4,548,862 (Hartman): "modern electronic devices are becoming so small and their electrical terminals are so delicate and closely spaced that it is difficult and expensive to make electrical connections by soldering or other established techniques. U.S. Pat. No. 4,113,981 (Fujita et al.) uses an adhesive layer for individually electrically interconnecting multiple pairs of arrayed electrodes. The adhesive layer includes spherical electrically conductive particles of substantially the same thickness as the adhesive, thus providing a conductive path through each particle that bridges facing pairs of electrodes. The particles are randomly distributed throughout the adhesive layer, but the Fujita patent indicates that if the particles comprise less than 30% by volume of the layer, they will be sufficiently spaced so that the intervening adhesive will insulate against short circuiting between laterally adjacent electrodes. Carbon powder, SiC powder and metal powder are said to be useful.
"U.S. Pat. No. 3,475,213 (Stow) discloses a tape having an electrically conductive backing and a pressure-sensitive adhesive layer which contains a monolayer of electrically conductive particles that could be identical to the adhesive layer of the Fujita patent if Fujita were to use a pressure-sensitive adhesive" (col. 1, lines 15-38 of the Hartman patent). The Stow patent says that the particles should "have a substantial thickness slightly less than the thickness of the adhesive film" (col. 3, lines 1-2), and "essentially none of the particles should extend above the surface of the adhesive to preserve satisfactory adhesion values" (col. 3, lines 39-41). The Stow patent indicates a preference for metal particles, preferably flattened to appropriate thickness before being added to an adhesive-coating mixture, while also suggesting the use of "[m]etallized plastic or glass beads or spheres" and that "the particles can be metal alloys, or composites in which one metal is coated on another" (col. 4, lines 52-55).
The Hartman patent concerns a flexible tape that, like the adhesive layer of the Fujita patent, can adhesively make individual electrical connections between multiple pairs of electrode arrays without short circuiting electrodes of either array by means of small particles that form electrically conductive bridges extending through the thickness of the layer. Each of the particles has a ferromagnetic core and an electrically conductive surface layer such as silver.
U.S. Pat. No. 4,606,962 (Reylek et al.), like the above-discussed patents, discloses a tape that provides electrical conductivity through the thickness of an adhesive layer, and is especially concerned with adhesively attaching a semiconductor die or chip to a substrate to dissipate both heat and static electricity. The adhesive layer of the Reylek tape, which preferably is heat-activatable, contains electrically and thermally conductive particles that at the bonding temperature of the adhesive are at least as deformable as are substantially pure silver spherical particles. The thickness of the particles exceeds the thickness of the adhesive between particles. When the particle-containing adhesive layer is removed from the carrier layer of the transfer tape and compressed between two rigid plates, the particles are flattened to the thickness of the adhesive between particles, thus providing small, flat conductive areas at both surfaces of the adhesive layer. The particles preferably are substantially spherical and made of a metal such as silver or gold or of more than one material such as "a solder surface layer and either a higher melting metal core such as copper or a nonmetallic core" (col. 4, lines 20-21).
The Reylek patent says: "To economize the use of the electrically conductive particles, they may be located only in segments of the novel adhesive transfer tape which are to contact individual electrical conductors" (col. 2, lines 39-42). One technique suggested for so locating the particles is outlined at col. 2, lines 42-55, where the patentee (1) forms a viscous adhesive coating, (2) renders areas of the coating substantially tack-free, (3) applies electrically conductive particles that adhere only to the viscous portions of the coating, and then (4) polymerizes the viscous areas to a substantially tack-free state. When the viscous areas remaining after step (2) are small, the electrically conductive particles are individually positioned in a predetermined pattern.